Lecture 24: Photosynthesis

Question 1

Photosynthesis

Answer 1

Synthesis of carbohydrates and oxygen in plants
Light dependent and light independent reactions
E + 6CO2 + 6H2O = C6H12O6 + 6O2

Question 2

Light reactions

Answer 2

Use light energy to oxidize water to oxygen and synthesize NADPH and ATP
Take place in thylakoid membrane: Photosystems 1 and 2
Excitation of reaction centres:
1. Oxidation of water to oxygen
2. Reduction of NADP to NADPH
Generation of transmembrane proton gradient that powers ATP synthesis

Question 3

Light-independent reactions

Answer 3

Convert CO2 to carbohydrates using NADPH and ATP

Occur within the stroma

Question 4

Photoreceptors

Answer 4

Variety to light absorbing groups: pigments
Cause different colour of a plant
Chlorophyll, beta-Carotene, Phycocyanin
Absorb light of different wavelengths
Absorbing a photon increases the potential energy of a pigment, energy can be released in different ways when the molecule returns to ground state

Question 5

Chlorophyll A

Answer 5

Absorbs red and blue light

We see plants as green

Question 6

Chlorophyll B

Answer 6

Absorbs red and blue light

We see plants as green

Question 7

Planck’s Law

Answer 7

Energy of light is inversely related to its wavelength

E=hc/wavelength

Question 8

Light-harvesting complexes

Answer 8

Membrane proteins that contain pigments
Protein environment influences the wavelength of the light that is absorbed
Energy from absorbed photons is transferred between neighbouring chlorophyll to a chlorophyll that acts as a reaction centre
Part of two different super complexes: photosystems 1 and 2

Question 9

Reaction centres

Answer 9

Primary reactions of photosynthesis occur at specific chlorophyll molecules called reaction centres

Question 10

Antennae

Answer 10

Other pigments that transfer absorbed light to reaction centre

Question 11

Photosystem II

Answer 11

Begins light reactions
In chloroplasts grant with little contact with stroma
Contains several light-absorbing pigments and redox-active cofactors as prothetic groups

Question 12

P680

Answer 12

Chlorophyll dimer
Reaction centre of PSII
Large absorption peak at wavelength 680
Oxidized form: P680+
Can only be reoxidized after is absorbs light: reduction potential changes dramatically when it absorbs a photon

Question 13

P680+

Answer 13

Within photosystem II
Strongest biological oxidizing agent known
Captures electrons from water, resulting in O2 and P680
Requires manganese containing cofactor
2H2O = O2 + 4H +4e

Question 14

P680*

Answer 14

Activated P680 by photon
Very negative reduction potential
Very easily loses an electron and is oxidized
Oxidized by PQ to P680+

Question 15

Plastoquinone

Answer 15

Accepts two protons and electron from P680* to make plastoquinol
For every H2O, two plastoquinone molecules are reduced
Electrons in plastoquinol are transferred to cytochrome b6f

Question 16

Cytochrome b6f

Answer 16

Resembles mitochondria complex III
Transfer of 4e to 4 plastocyanin (Cu2+) to give 4 plastocyanin (Cu1+) - reduction
Regeneration of PQ
Pumping of protons into thylakoid lumen

Question 17

Photosystem I

Answer 17

Super complex containing light harvesting complexes located in individual (unstacked) storm lamella
Contains chlorophyll and carotenoids
Chlorophyll dimers form reactions centres: P700

Question 18

P700

Answer 18

Absorption of wavelength 700nm
Generates high-energy electrons in response to light energy, which are transferred to ferredoxin (reduced)
Reduced to original state by plastocyanin from cytochrome b6f complex
Final outcome: recution of NADP+ to NADPH

Question 19

Ferredoxin

Answer 19

Accepts electrons from excited P700 reaction centre

Transfers electrons to ferredoxin-NADP+ reductase

Question 20

Ferredoxin-NADP+ reductase

Answer 20

Accepts electrons from ferredoxin
Reduces NADP+ to NADPH
NADPH is final acceptor of electrons removed from H20 in PSII

Question 21

P700+

Answer 21

Reduced to original state by accepting electrons from plastocyanin

Question 22

Photophosphorylation

Answer 22

Chloroplasts use the same mechanism as mitochondria to synthesize ATP
ATP produces in stroma

Question 23

Cyclic electron flow

Answer 23

Between PSI and cytochrome b6f leads to proton pumping without generation of NADPH or oxygen
Does not require PSII
Light energy harvested by PSI fuels the proton pumping activity of cytochrome b6f

Question 24

Dark reactions

Answer 24

Calvin cycle, light-independent reactions
Utilize the products of light reactions
Occur in chloroplast storm and fix atmospheric CO2 into carbohydrates
Can occur in light but do not light themselves
Require ATP and NADPH

Question 25

CO2 fixation

Answer 25

Synthesis of carbohydrates from CO2
Catalyzed by enzyme Rubisco
Rate limiting step

Question 26

Rubisco

Answer 26

Ribulose bisphosphate carboxylase/oxygenase

Carboxylates 5-carbon sugar and cleaves it into two 3C sugars

Question 27

Glyceraldehyde-3-phosphate dehydrogenase

Answer 27

Reduction of C3 glycerates
To two glyceraldehyde-3-phosphates
Synthesis of hexoses

Question 28

Phosphoglycerate kinase

Answer 28

Phosphorylates 3-phosphoglycerate (produce of rubisco) to 1,3 bisphosphoglycerate
2 ATP required

Question 29

Photorespiration

Answer 29

Rubisco acts as an oxygenate
Uses oxygen instead of CO to generate 2C molecule phosphoglycolate and 3-PG instead of 2 molecules of 3-PG
Some phosphoglycolate can eventually be converted back to RuBP: costs ATP and NADPH and produces CO2
Occurs at low CO2 levels and high temperatures